The Asterisk Extension Language (AEL) in pbx/pbx_ael.c in Asterisk does not properly generate extensions, which allows remote attackers to execute arbitrary extensions and have an unknown impact by specifying an invalid extension in a certain form.
Asterisk Open Source 1.2.26 through 1.2.30.3 and Business Edition B.2.3.5 through B.2.5.5, when realtime IAX2 users are enabled, allows remote attackers to cause a denial of service (crash) via authentication attempts involving (1) an unknown user or (2) a user using hostname matching.